System and Method for WLAN OFDMA Design of Uplink and Downlink Transmissions

ABSTRACT

Embodiments are provided for WLAN OFDMA operations in uplink and downlink transmissions. For OFDMA downlink transmission, an access point (AP) sends a signaling requesting protection of a transmission channel from access by stations (STAs) associated with the AP. The AP then sends an OFDMA physical layer Protocol Data Unit (PPDU) and a Block Acknowledgement Request (BAR) to the STAs, and receives a Block Acknowledgement (BA) from each one of the STAs. For OFDMA uplink transmissions, an STA sends a signaling requesting protection of a transmission channel from access by the AP. The STA then sends, to the AP, a frame indicating start of OFDMA transmission by the STA, and receives, from the AP, an OFDMA Begin Receive (OBR) frame, which is sent by the AP to participating STAs. The STA then sends an OFDMA PPDU and a BAR to the AP, and receives a BA in return.

TECHNICAL FIELD

The present invention relates to network communications, and, inparticular embodiments, to a system and method for WLAN OFDMA design ofuplink and downlink transmissions.

BACKGROUND

Wireless Local Area Networks (WLANs) commonly operate in unlicensedspectrum bands. Rules for operation in these bands force competingdevices to share the available resources and defer their intendedtransmissions when the medium is sensed busy. The medium refers to thetransmission channel or bandwidth shared by multiple components of theWLAN to transmit uplink and downlink signals. The bandwidth (orfrequency range) is shared for uplink and downlink transmissions.Typically, a WLAN uses an Orthogonal Frequency-Division Multiplexing(OFDM) transmission format in which all transmission resources areassigned to a single device. Random assignment is commonly achievedusing carrier sense multiple access with collision avoidance (CSMA/CA).With CSMA/CA a device wins access to the medium, transmits its data upto a pre-defined period of time, and then gives up the medium for otherdevices to compete for transmission. In contrast, Orthogonal FrequencyDivision Multiple Access (OFDMA) is a transmission and access mechanismthat accommodates multiple-user transmissions simultaneously. OFDMA iscommonly implemented in wireless infrastructures operating in thelicensed bands in order to meet timing information in terms of framestructure and the scheduling of resources among a subset of users. Thereis a need for efficient schemes for implementing OFDMA in WLANs.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a method by an access point (AP) forOrthogonal Frequency Division Multiple Access (OFDMA) downlinktransmission in a Wireless Local Area Network (WLAN) includes sending asignaling requesting protection of a transmission channel for downlinktransmissions from access by stations (STAs) associated with the AP. Themethod further includes sending an OFDMA physical layer Protocol DataUnit (PPDU) to the STAs, and sending a Block Acknowledgement Request(BAR) to the STAs. In return, a Block Acknowledgement (BA) is receivedfrom each one of the STAs.

In accordance with another embodiment, a method by an STA for OFDMAdownlink transmission in a WLAN includes detecting a signaling from anAP. The signaling indicates the AP requesting protection of atransmission channel from access by a plurality of STAs associated withthe AP including the STA. The method further includes receiving, fromthe AP an OFDMA PPDU, and receiving, from the AP, a BAR. The BAR is sentto all participating STAs. In return, the STA sends a BA to the AP.

In accordance with another embodiment, a method by a STA for OFDMAuplink transmissions in a WLAN includes sending a signaling requestingprotection of a transmission channel from access by an AP of the STA.The method further includes sending, to the AP, a frame indicating startof OFDMA transmission by the STA, and receiving, from the AP, an OFDMABegin Receive (OBR) frame. The OBR frame is sent by the AP to aplurality of STAs associated with the AP including the STA. The STA thensends, to the AP, an OFDMA PPDU and a BAR after the OFDMA PPDU. Inreturn, the STA receives a BA from the AP.

In accordance with another embodiment, a method by an AP for OFDMAuplink transmission in a WLAN includes detecting a signaling from a STA.The signaling indicates the STA requesting protection of a transmissionchannel from access by the AP. The method further includes receiving,from the STA, a frame indicating start of OFDMA transmission by the STA,and sending, to a plurality of STAs associated with the AP including theSTA, an OBR frame. The AP then receives, from the STA, an OFDMA PPDUfollowed by a BA, and sends to the STA with a BA.

In accordance with another embodiment, an AP supporting OFDMAtransmission in a WLAN comprises a processor and a computer readablestorage medium storing programming for execution by the processor. Theprogramming includes instructions to send a signaling requestingprotection of a transmission channel from access by STAs associated withthe AP, and send an OFDMA PPDU to the STAs. The programming includesfurther instructions to send a BAR to the STAs, and receive a BA fromeach one of the STAs.

In accordance with yet another embodiment, a user station supportingOFDMA transmission in a WLAN comprises a processor and a computerreadable storage medium storing programming for execution by theprocessor. The programming includes instructions to send a signalingrequesting protection of a transmission channel from access by an AP ofthe user station, and send, to the AP, a frame indicating start of OFDMAtransmission by the STA. The programming includes further instructionsto receive, from the AP, an OBR frame which is sent by the AP to aplurality of STAs associated with the AP, and send, to the AP, an OFDMAPPDU. The programming further configure the user station to send a BARto the AP, and receive a BA from the AP.

The foregoing has outlined rather broadly the features of an embodimentof the present invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates a WLAN basic service set (BSS);

FIG. 2 illustrates embodiment of events during a downlink (DL) OFDMATXOP;

FIG. 3 illustrates a Block Acknowledgement Request (BAR) frame format;

FIG. 4 illustrates a BAR control field format;

FIG. 5 illustrates embodiment of events during an uplink (UL) OFDMATXOP;

FIG. 6 illustrates an embodiment of UL OFDMA transmissions;

FIG. 7 illustrates an embodiment method of AP and STAs operations duringthe DL OFDMA TXOP;

FIG. 8 illustrates an embodiment method of AP and STAs operations duringthe UL OFDMA TXOP; and

FIG. 9 is a diagram of a processing system that can be used to implementvarious embodiments.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

Disclosed herein are system and method embodiments for WLAN OFDMAoperations in uplink and downlink transmissions. FIG. 1 shows an exampleof a WLAN basic service set (BSS) including an access point (AP) and oneor more stations (STAs). The AP is a communication device that allowsthe STAs to access and communicate with the WLAN. The STAs are any usercommunication devices allowing users or subscribers to communicate withthe AP and hence the WLAN. Examples of STAs include smartphones, tabletcomputers, laptop computers, desktop computers, sensor devices (e.g.,smartwatch), and other mobile or communication devices with WLAN (e.g.,WiFi) capability.

The STAs and the AP compete for access to a shared medium, e.g., sharedwireless channel bandwidth, symbols, or subcarriers. Access to themedium may be based on random access mechanisms such as carrier sensemultiple access with collision avoidance (CSMA/CA) or using a schedulingmechanism that allows the STAs and AP access to the medium at specifiedor predetermined times. Irrespective of how access to the medium isobtained, a downlink (DL) OFDMA transmission opportunity (TXOP) isobtained when the AP has the right to access the medium and communicateswith multiple STAs in a single OFDMA transmission. An uplink (UL) OFDMATXOP is obtained when a STA gains access to the medium and is ready tostart an UL OFDMA TXOP by inviting other STAs to share the availabletransmission resources, e.g. the subcarriers.

In general, an OFDMA system consists of an integer N_(sc) of subcarriersdefined in a channel of certain bandwidth, W Megahertz (MHz). Forexample in WLAN, the bandwidth W is usually set to 20 MHz. Thesubcarrier separation Δf is given by Δf=W/N_(sc). The OFDMA symbolduration T_(s) is given by 1/Δf. The quantity N_(sc) is set at 64 inWLAN OFDM implementation. With the introduction of OFDMA to WLAN, N_(sc)may be set to a higher value, such as 256 or 512 to achieve finergranularity. During each OFDMA transmission (uplink or downlink), anumber of subcarriers are allocated to each of the users participatingin the transmission.

FIG. 2 shows a sequence of events during a DL OFDMA TXOP according to anembodiment of the disclosure. Using CSMA/CA, a DL OFDMA TXOP starts whenthe AP gains access to the medium. The first action that the AP performsis to protect the medium. The protection of the medium can be performedby a signaling mechanism to request protection of the medium form accessbother entities. In an embodiment, the signaling mechanism is sending aclear-to-send (CTS). As such, a CTS frame is sent by the AP using anon-High Throughput (non-HT) format and comprises a time value in aLegacy Signal Field (L-SIG) in the CTS frame for the legacy STAs, or ina Duration/ID field of the CTS frame for non-OFDMA STAs (e.g., highthroughput (HT) and very high throughput (VHT) STAs). The time valueallows all STAs to defer their transmissions by an amount of time. Invarious embodiments, the CTS sent by the AP does not require a replyback to the AP from other entities (STAs) receiving this CTS.

Alternatively, the AP can protect the medium by sending arequest-to-send (RTS) message to one of its associated STAs. The STAthen responds with a CTS message back to the AP. The RTS and CTSmessages are sent using non-HT format. The AP then generates an OFDMAphysical layer (PHY) Protocol Data Unit (PPDU) that includes framesdestined to a number of STAs. For instance, the recipient STAs can beidentified by a group ID (GrpID) in the OFDMA SIG field. The OFDMAresource allocation portion in the SIG field may be determined by anyprevious suitable packet transmission from the AP to the STAs, insteadof using any separate channel sounding for the channel responseinformation per each STA. For example, packets that were used for thecontention to access the medium by each STA can also be used to get thechannel response of each STA and to determine OFDMA resource allocationportion in the SIG field.

When the transmission of the OFDMA PPDU is finished, the AP requestsacknowledgements from those STAs participating in the OFDMAtransmission. Specifically, the AP sends Block Acknowledgement Request(BAR) frames using OFDMA, one frame for each of the participating STAs.Each STA responds with a Block ACK (BA) frame according to one of thefollowing scenarios. If UL OFDMA is supported, then the participatingSTAs use OFDMA to send their BAs in the uplink direction. In this case,timing information for synchronizing the uplink transmissions needs tobe propagated from the AP to the STAs. The timing information may beincluded in the BAR frame by extending the BAR information field. FIG. 3shows a BAR frame format. The BAR information field in the frame is ofvariable length and suitable for adding timing information at the startor at the end of the field.

Alternatively, if UL OFDMA is not supported, then the participating STAsneed to send their BA frames in sequence. One way to determine thesequence for sending the BA frames is for the AP to transmit the BARframes one after the other using non-HT format (e.g., without usingOFDMA format). However, this approach produces some overhead. Instead,the AP can send an explicit sequence using some of the reserved bits inthe BAR control field in the BAR frame. FIG. 4 shows a BAR control fieldformat. The number of bits needed to indicate the sequence depends onthe maximum number of STAs that are allowed to participate in the OFDMAtransmission. For example, in the case of 4 STAs, 2 bits are needed forthe AP to communicate to the STAs the expected sequence of BA frames.

FIG. 5 shows a sequence of events during an UL OFDMA TXOP according toan embodiment of the disclosure. The UL OFDMA TXOP starts when anOFDMA-capable STA (non-AP STA) acquires the medium. Similar to the DLOFDMA TXOP, the first action of the STA is to protect the medium by asignaling mechanism. The signaling mechanism requests protection of themedium from access by the AP. The signaling comprises sending an RTSframe to the AP and wait for the CTS response. The STAs defer theirtransmissions based on information included in the RTS/CTS frames.

The STA then sends an OFDMA Begin Transmission (OBX) frame to the AP.The reception of the OBX frame by the AP is an indication that the STAwishes to start an UL OFDMA TXOP by sharing the medium with other STAs.The AP selects other STAs to participate in the UL OFDMA, e.g., STAx andSTAy in FIG. 5, based on knowledge available at the AP on trafficrequirements and frame availability. The AP then responds with an OFDMABegin Receive (OBR) frame to participating STAs. In addition to theidentities of stations participating in the UL TXOP, the OBR frame mayinclude information such as timing information, the allocation ofsubcarriers to STAs using, for example, GrpID and subcarrier groupindex, the maximum PPDU length that STAs must adhere to, and/or anyother relevant information. The OBR frame acts as a trigger to start theUL transmissions by participating stations. Each STA may compensate thetiming offset and carrier frequency offset using Legacy STFs and LTFsappended in the OBR frame. Before the participating STAs, e.g., STAx andSTAy in FIG. 5, transmit the UL frame, OFDMA PPDU, a time delay isneeded after the OBR reception to allow enough processing time for eachparticipating STA to switch from receiving mode to transmitting mode.For instance, each STA waits at least a 30 microseconds to establish anInter Frame Spacing between the OBR reception and the OFDMA PPDUtransmission. The Inter Frame Spacing avoids transient frequencyagitation which can occur during the STA switching from the receivingmode to transmitting mode.

FIG. 6 shows UL OFDMA transmissions according to an embodiment of thedisclosure. After performing the synchronization during the OBX/OBRexchange of the UL OFDMA TXOP, the STAs start their OFDMA transmissionsof PPDUs, as described above. The participating STAs transmit theircorresponding PPDUs simultaneously on the uplink to the AP using theirallocated subcarriers. After the successful transmission of the PPDUs byparticipating STAs, the AP can send BA frames after receiving BAR framestransmitted by the participating STAs using OFDMA format. The BA framesmay be sent by the AP using the OFDMA format or can be sent sequentiallyas shown in FIG. 5.

FIG. 7 shows an embodiment method 700 of AP operations during a DL OFDMATXOP. At step 710, the AP gains access to the medium, e.g., usingCSMA/CA. At step 720, the AP sends a signaling requesting protection ofthe medium from access by the STAs. The signaling is sent using a CTSframe or using a RTS message to each STA. The CTS frame allows all STAsto defer their transmissions by an amount of time. In variousembodiments, the CTS sent by the AP does not require a reply back to theAP from other entities (STAs) receiving this CTS. If a RTS message issent, the STA responds by a CTS message back to the AP. At step 730, theAP sends an OFDMA PPDU to STAs, e.g., identified by a GrpID in an OFDMASIG field. At step 740, the AP sends a BAR for the STAs using OFDMA. Atstep 750, the AP receives a BA frame from each STA using OFDMA or insequence (e.g., as determined by the BARs).

FIG. 8 shows an embodiment method of STAs operations during an UL OFDMATXOP. At step 810, the STA gains access to the medium, e.g., usingCSMA/CA. At step 820, the STA sends a signaling requesting protection ofthe medium from access by the AP. The signaling is sent using a RTSmessage to the AP. At step 830, the STA receives a CTS from the AP. Atstep 840, the STA sends an OBX frame to the AP. At step 850, the STAreceives, with other STAs selected by the AP to share the medium forOFDMA uplink transmissions, an OBR from the AP. At step 860, the STAssend OFDMA PPDUs to the AP. At step 870, each STA sends a BAR to the AP.At step 880, the STAs receive BAs from the AP.

FIG. 9 is a block diagram of a processing system 900 that can be used toimplement various embodiments. For instance the processing system 900can be part of a AP, a STA, or a controller in a WLAN. Specific devicesmay utilize all of the components shown, or only a subset of thecomponents, and levels of integration may vary from device to device.Furthermore, a device may contain multiple instances of a component,such as multiple processing units, processors, memories, transmitters,receivers, etc. The processing system 900 may comprise a processing unit901 equipped with one or more input/output devices, such as a speaker,microphone, mouse, touchscreen, keypad, keyboard, printer, display, andthe like. The processing unit 901 may include a central processing unit(CPU) 910, a memory 920, a mass storage device 930, a video adapter 940,and an I/O interface 960 connected to a bus. The bus may be one or moreof any type of several bus architectures including a memory bus ormemory controller, a peripheral bus, a video bus, or the like.

The CPU 910 may comprise any type of electronic data processor. Thememory 920 may comprise any type of system memory such as static randomaccess memory (SRAM), dynamic random access memory (DRAM), synchronousDRAM (SDRAM), read-only memory (ROM), a combination thereof, or thelike. In an embodiment, the memory 920 may include ROM for use atboot-up, and DRAM for program and data storage for use while executingprograms. In embodiments, the memory 920 is non-transitory. The massstorage device 930 may comprise any type of storage device configured tostore data, programs, and other information and to make the data,programs, and other information accessible via the bus. The mass storagedevice 930 may comprise, for example, one or more of a solid statedrive, hard disk drive, a magnetic disk drive, an optical disk drive, orthe like.

The video adapter 940 and the I/O interface 960 provide interfaces tocouple external input and output devices to the processing unit. Asillustrated, examples of input and output devices include a display 990coupled to the video adapter 940 and any combination ofmouse/keyboard/printer 970 coupled to the I/O interface 960. Otherdevices may be coupled to the processing unit 901, and additional orfewer interface cards may be utilized. For example, a serial interfacecard (not shown) may be used to provide a serial interface for aprinter.

The processing unit 901 also includes one or more network interfaces950, which may comprise wired links, such as an Ethernet cable or thelike, and/or wireless links to access nodes or one or more networks 980.The network interface 950 allows the processing unit 901 to communicatewith remote units via the networks 980. For example, the networkinterface 950 may provide wireless communication via one or moretransmitters/transmit antennas and one or more receivers/receiveantennas. In an embodiment, the processing unit 901 is coupled to alocal-area network or a wide-area network for data processing andcommunications with remote devices, such as other processing units, theInternet, remote storage facilities, or the like.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A method by an access point (AP) for OrthogonalFrequency Division Multiple Access (OFDMA) downlink transmission in aWireless Local Area Network (WLAN), the method comprising: sending asignaling requesting protection of a transmission channel for downlinktransmissions from access by stations (STAs) associated with the AP;sending an OFDMA physical layer Protocol Data Unit (PPDU) to the STAs;sending a Block Acknowledgement Request (BAR) to the STAs; and receivinga Block Acknowledgement (BA) from each one of the STAs.
 2. The method ofclaim 1, further comprising gaining access to the transmission channelbefore sending the signaling.
 3. The method of claim 1, wherein sendingthe signaling requesting protection of the transmission channel fromaccess by the STAs includes sending by the AP a clear-to-send (CTS)frame, wherein the CTS frame is sent using a non-High Throughput(non-HT) format and comprises timing information allowing the STAs todefer their transmissions by an amount of time.
 4. The method of claim3, wherein the timing information is sent in one of a Legacy SignalField (L-SIG) of the CTS frame for legacy STAs and a Duration/ID fieldof the CTS frame for non-OFDMA STAs.
 5. The method of claim 1, whereinsending the signaling requesting protection of the transmission channelfrom access by stations (STAs) includes: sending a request-to-send (RTS)message to the STAs, wherein the RTS message comprises timinginformation allowing the STAs to defer their transmissions by an amountof time; and receiving a clear-to-send (CTS) message from the STAs. 6.The method of claim 5, wherein the RTS message and the CTS message aresent using non-High Throughput (non-HT) format.
 7. The method of claim1, wherein the OFDMA PPDU comprises a Signaling (SIG) field including agroup ID that identifies recipient STAs for the OFDMA PPDU, whereinOFDMA resource allocation in the SIG field is determined by a previouspacket transmission from the AP.
 8. The method of claim 1, wherein theSTAs support OFDMA uplink transmission, wherein the BAR comprises a BARinformation field of variable length, the BAR information fieldincluding timing information for synchronizing OFDMA uplinktransmissions from the STAs, and wherein the BA is sent by the STAssimultaneously using OFDMA.
 9. The method of claim 1, wherein the STAsdo not support OFDMA uplink transmission, wherein the BAR is sent usingnon-High Throughput (non-HT) format and comprises a BAR control field ofvariable number of bits according to a maximum number of the STAs, theBAR control field indicating a sequence of transmissions to the STAs,and wherein the BA is sent by the STAs according to the sequence oftransmissions.
 10. A method by a station (STA) for Orthogonal FrequencyDivision Multiple Access (OFDMA) downlink transmission in a WirelessLocal Area Network (WLAN), the method comprising: detecting a signalingfrom an access point (AP), the signaling indicating the AP requestingprotection of a transmission channel from access by a plurality of STAsassociated with the AP including the STA; receiving, from the AP anOFDMA physical layer Protocol Data Unit (PPDU); receiving, from the AP,a Block Acknowledgement Request (BAR), wherein the BAR is sent to theSTAs; and sending, to the AP, a Block Acknowledgement (BA).
 11. Themethod of claim 10, wherein detecting the signaling from the APcomprises detecting a clear-to-send (CTS) frame by the AP, wherein theCTS frame is sent using a non-High Throughput (non-HT) format andcomprises timing information allowing the STAs to defer transmission byan amount of time.
 12. The method of claim 10, wherein detecting thesignaling from the AP comprises receiving from the AP, a request-to-send(RTS) message that is sent to the STAs, wherein the RTS messagecomprises timing information allowing the STAs to defer transmission byan amount of time, and wherein the method further comprises sending, tothe AP, a clear-to-send (CTS) message in response to the RTS message.13. A method by a station (STA) for Orthogonal Frequency DivisionMultiple Access (OFDMA) uplink transmissions in a Wireless Local AreaNetwork (WLAN), the method comprising: sending a signaling requestingprotection of a transmission channel for uplink transmissions fromaccess by an access point (AP) of the STA; sending, to the AP, a frameindicating start of OFDMA transmission by the STA; receiving, from theAP, an OFDMA Begin Receive (OBR) frame, wherein the OBR frame is sent bythe AP to a plurality of STAs associated with the AP including the STA;sending, to the AP, an Orthogonal Frequency Division Multiple Access(OFDMA) physical layer Protocol Data Unit (PPDU); sending a BlockAcknowledgement Request (BAR) to the AP; and receiving a BlockAcknowledgement (BA) from the AP.
 14. The method of claim 13, furthercomprising gaining access to the transmission channel before sending thesignaling.
 15. The method of claim 13, wherein sending the signalingrequesting protection of the transmission channel from the AP includes:sending, to the AP, a request-to-send (RTS) message, wherein the RTSmessage comprises time information allowing the AP to defer transmissionby an amount of time; and receiving, from the AP, a clear-to-send (CTS)message, wherein the CTS message is sent by the AP to the STAs.
 16. Themethod of claim 15, wherein the OBR frame includes identities of STAsselected for OFDMA uplink transmissions and optionally at least one oftiming information for the STAs for synchronizing OFDMA transmissionsfrom the STAs, allocation of a plurality of subcarrier groups to theSTAs for OFDMA transmissions, and a maximum PPDU length for the STAs.17. The method of claim 16, wherein the OBR comprises a group IDdesignating the STAs, and a plurality of subcarrier group indicesindicating the subcarrier groups.
 18. The method of claim 16, whereinthe OFDMA PPDU is sent to the AP on one or more subcarriers from thesubcarrier groups allocated to the STA.
 19. The method of claim 13,before sending the OFDMA PPDU, waiting an amount of time providing anInter Frame Spacing between receiving the OBR frame and sending theOFDMA PPDU, wherein the Inter Frame Spacing allows the STA to switchfrom receiving mode to transmitting mode.
 20. A method by an accesspoint (AP) for Orthogonal Frequency Division Multiple Access (OFDMA)uplink transmission in a Wireless Local Area Network (WLAN), the methodcomprising: detecting a signaling from a station (STA), the signalingindicating the STA requesting protection of a transmission channel fromaccess by the AP; receiving, from the STA, a frame indicating start ofOFDMA transmission by the STA; sending, to a plurality of STAsassociated with the AP including the STA, an OFDMA Begin Receive (OBR)frame; receiving, from the STA, an OFDMA physical layer Protocol DataUnit (PPDU); receiving, from the STA, a Block Acknowledgement Request(BAR); and sending, to the STA, a Block Acknowledgement (BA).
 21. Themethod of claim 20, wherein detecting the signaling from the STAincludes: receiving, from the STA, a request-to-send (RTS) message,wherein the RTS message comprises time information allowing the AP todefer transmission by an amount of time; and sending, to the STAs, aclear-to-send (CTS) message.
 22. The method of claim 20, wherein the OBRframe includes allocation of a plurality of subcarrier groups to theSTAs for OFDMA transmissions, and wherein the method further comprisesreceiving from the STAs simultaneously a plurality of OFDMA PPDUs onrespective subcarriers of the STAs according to the allocation of thesubcarrier groups to the STAs.
 23. The method of claim 20 furthercomprising, upon receiving the OBR frame from the STA, selecting otherSTAs to participate with the STA in the OFDMA uplink transmission. 24.An access point (AP) supporting Orthogonal Frequency Division MultipleAccess (OFDMA) transmission in a Wireless Local Area Network (WLAN), theAP comprising: a processor; and a computer readable storage mediumstoring programming for execution by the processor, the programmingincluding instructions to: send a signaling requesting protection of atransmission channel from access by stations (STAs) associated with theAP; send an Orthogonal Frequency Division Multiple Access (OFDMA)physical layer Protocol Data Unit (PPDU) to the STAs; send a BlockAcknowledgement Request (BAR) to the STAs; and receive a BlockAcknowledgement (BA) from each one of the STAs.
 25. The AP of claim 24,wherein the programming includes further instructions to: detect asignaling from a STA, the signaling indicating the STA requestingprotection of a transmission channel for uplink transmissions fromaccess by the AP; receive, from the STA, a frame indicating start ofOFDMA transmission by the STA; send, to the STAs including the STA, anOFDMA Begin Receive (OBR) frame; receive, from the STAs, OFDMA physicallayer Protocol Data Units (PPDUs); receive, from the STAs, BlockAcknowledgement Requests (BARs); and send, to the STAs, BlockAcknowledgements (BAs).
 26. A user station supporting OrthogonalFrequency Division Multiple Access (OFDMA) transmission in a WirelessLocal Area Network (WLAN), the user station comprising: a processor; anda computer readable storage medium storing programming for execution bythe processor, the programming including instructions to: send asignaling requesting protection of a transmission channel from access byan access point (AP) of the user station; send, to the AP, a frameindicating start of OFDMA transmission by the STA; receive, from the AP,an OFDMA Begin Receive (OBR) frame, wherein the OBR frame is sent by theAP to a plurality of STAs associated with the AP including the userstation; send, to the AP, an OFDMA physical layer Protocol Data Unit(PPDU); send a Block Acknowledgement Request (BAR) to the AP; andreceive a Block Acknowledgement (BA) from the AP.
 27. The user stationof claim 26, wherein the programming includes further instructions to:detect a signaling from the AP, the signaling indicating the APrequesting protection of a transmission channel for downlinktransmissions from access by the STAs including the user station;receive, from the AP an OFDMA PPDU; receive, from the AP, a BAR, whereinthe BAR is sent to the STAs; and send, to the AP, a BA.